Yeah, I think a single postgres cluster with read replicas should be able to handle the needs of a single instance just fine. You can then horizontally scale the backend and frontend to keep up using containers.

Ideally, that instance can scale up to a few million users, and then federation can provide the horizontal scaling that takes the lemmyverse up to Reddit scale.

The backend just needs to handle databases better. Adding support for read replicas, making it more efficient, etc.

Not sure how well pict-rs scales, but that's probably pretty light already, vertical scaling might be good enough that it'll always be limited by the DB.

But yeah, I guess the worst case scenario is that Postgres doesn't scale enough and we need to switch to something like cockroachdb. Or go for snowflake uuids and noSQL like Twitter did back in the day.